Uniform thermomigration utilizing sample movement

ABSTRACT

The geometric configuration of a molten zone migrating through a solid body of semiconductor material during thermal gradient zone melting is maintained by noncentro-symmetric rotation of the solid body about an axis displaced therefrom, by centro-symmetric rotation of the solid body, by translation of the solid body, by a combination of centro-symmetric rotation and translation of the solid body or by a combination of noncentro-symmetric rotation and translation of the solid body while being heated by a suitable heat source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the migration of a molten zone through a solidbody of semiconductor material by thermal gradient zone melting.

2. Description of the Prior Art

W. G. Pfann described in "Zone Melting", John Wiley and Sons, Inc., NewYork (1966), a thermal gradient zone melting process to produce variousdesirable material configurations in a body of semiconductor material.The process had previously been disclosed in his issued U.S. Pat. No.2,813,048, based on his application filed June 24, 1954. In bothinstances, cavities are generally formed in the surface of the body anda piece of wire of the metal to be migrated is disposed on the cavity.However, the resulting structures were not desirable for semiconductorusage.

M. Blumenfeld, in U.S. Pat. No. 3,897,277, teaches alloying aluminum tothe surface of the body of silicon semiconductor material in an attemptto maintain the registry of the pattern of metal deposits to bemigrated. However, problems of precise registry of the metal stillplague one's attempt to obtain the precision necessary to obtain anarray of deep diodes suitable for making x-ray imaging devices.

Recently, Thomas R. Anthony and Harvey E. Cline, discovered thatemploying selective chemical etching of the surface and preferredcrystallographic orientation of the surface and molten zones enabled oneto employ thermal gradient zone melting processing to assist in makingsemiconductor devices commercially feasible. The improved processresulted in a large savings in energy required to process semiconductormaterials and increased yields. For a further teaching of the improvedprocess, one is directed to their teachings in their recently grantedU.S. Pat. No. 3,904,442, and copending patent application Ser. No.519,913 filed Nov. 1, 1974, now U.S. Pat. No. 3,979,230 assigned to theassignee of the present invention and incorporated herein by reference.

To practice thermal gradient zone melting on a commercial basis, oneshould make the process as simple as possible. Consequently, John Boahin his copending U.S. patent application Ser. No. 578,807 filed May 19,1975, and now U.S. Pat. No. 4,001,047 describes the use of radiantenergy as a source for the migration or movement of a molten zonethrough a solid body. This application is assigned to the assignee ofthe present invention and is incorporated herein by reference. Theprocess and apparatus arrangement taught by Boah is very practical forthe manufacture of many devices. In the manufacture of certainsemiconductor devices, the maintenance of a temperature gradientprecisely in a direction perpendicular to a major surface of thesemiconductor body being treated is desirable. Maintenance of such atemperature gradient and the elimination of lateral or transversetemperature gradients ensures the precise orientation of junctionswithin the devices and maximization of manufacturing yields.

One source of radiant energy which is presently being employed inpracticing temperature gradient zone melting is quartz lamps havingtungsten filaments. A heat source may comprise a plurality of such lampsoriented in a parallel array. However, due to the construction the lampsand the spacing of the lamps from each other within the array,temperature irregularities as great as 40° C over distances ofapproximately 1 cm have been observed in the area of illumination ofsuch an array. Such temperature irregularities contribute to lateraltemperature gradients which adversely affect the device beingmanufactured in a manner hereinafter described.

Direct observation of such an array of lamps when energized hasindicated that the temperature irregularities stemming from theconstruction of the lamps are due to a self-shadowing of the coiledtungsten filament, the shadowing of the disk shaped filament holdersdisposed within the quartz envelope, and the refractive properties ofthe quartz envelopes. The spacing of the lamps and thus the tungstenfilaments within the array also contributes to the non-uniformities intemperature of the radiation emitted by the lamps. Moreover,non-uniformities in reflection of any reflectors employed with thequartz lamps further contributes to the irregularities in temperatureexperienced by the semiconductor bodies heated by the lamps and thusenhances the creation of lateral temperature gradients. Additionally,any quartz plates disposed between the lamps and the semiconductorbodies such as, for example, a quartz convection suppressor plate or aquartz coverplate for an air cooling channel surrounding the lamps maypossess refractive properties which may further contribute totemperature irregularities and the creation of lateral temperaturegradients.

During the migration of the molten zone through the semiconductormaterial, any lateral temperature gradients distort the movement of themolten zone. That is, the geometrical configuration of the regionsproduced by the movement of the molten zone through the solid bodychanges in accordance with the thermal gradients, both lateral normal,in the region of movement. The lateral thermal gradient is of particularconcern in that when forming grid structures, for electrical isolationof devices, distortion of the array forming the grid due to theselateral gradients may be so great as to eliminate the outer 2 mmperipheral portion of a wafer from being usable. The geometrical patternof the grid structure is very distorted. Separation of isolation regionsoccur and surface tension pulls intersecting regions apart duringmigration.

It is therefore an object of this invention to provide a new andimproved thermal gradient zone melting process which overcomes thedeficiencies of the prior art.

Another object of this invention is to provide a new and improvedthermal gradient zone melting process wherein any lateral and/or radialtemperature gradients in a body or wafer of semiconductor material beingprocessed thereby is minimized.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereafter.

In accordance with the teachings of this invention, there is provided anew and improved thermal gradient zone melting method for migrating amolten zone through a solid body of semiconductor material. The methodcomprises the process steps of selecting a body of single crystalsemiconductor material having two major opposed surfaces which are,respectively, the top and bottom surfaces thereof. The body has apredetermined type conductivity, a predetermined level of resistivity, apreferred diamond crystal structure, a preferred planar crystalorientation for at least the top surface, and a first preferred crystalaxis and a central axis, which are each substantially perpendicular tothe top surface and substantially parallel with each other. A layer ofmetal of a predetermined thickness and a predetermined geometricalconfiguration is preferably vapor deposited on the major surface havingthe preferred planar crystal orientation. The processed body is placedon a support and rotated noncentro-symmetrically about an axis displacedfrom the central axis of the body, rotated centro-symmetrically,translated, moved by a combination of centro-symmetric rotation andtranslation or moved by a combination of noncentro-symmetric rotationand translation.

The body and the deposited metal are heated to a preselected elevatedtemperature sufficient to form a melt of a metal-rich semiconductormaterial on the surface of the body while continuing any one of theaforementioned movements of the body. A temperature gradient isestablished across the body substantially parallel with the central axisof the body and the first preferred crystal axis of the crystalstructure while continuing the movement of the body. The surface onwhich the melt is formed is retained at the lower temperature.Thereafter, each melt of metal-rich semiconductor material is migratedas a molten zone through the solid body of semiconductor material for asufficient period of time to reach a predetermined distance into thebody from the surface on which the melt is formed. The movement of thebody is continued during this migration. A region of recrystallizedsemiconductor material of the body having solid solubility of thedeposited metal therein is formed in the body by each melt. Each regionso produced has a predetermined geometric configuration, a substantiallyuniform cross-sectional area and a substantially uniform level ofresistivity throughout the entire region.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diamond cubic crystal structure.

FIG. 2 is a side elevation view, partly in cross-section of a body ofsemiconductor material processed in accordance with the teachings ofthis invention.

FIG. 3 illustrates a heat source and a suitable means for supporting thebody of FIG. 2 and rotating the body noncentro-symmetrically duringfurther processing.

FIG. 4 is a top planar view illustrating the noncentro-symmetricrotation of a plurality of the bodies of FIG. 2 in accordance with theteachings of this invention.

FIG. 5 illustrates a suitable means for supporting the body of FIG. 2and rotating the body centro-symmetrically during further processing.

FIG. 6 is a top planar view partially broken away of thecentro-symmetric rotation of a plurality of the bodies of FIG. 2 inaccordance with the teachings of this invention and one type of heatsource which may be used to heat the bodies.

FIG. 7 illustrates a suitable means for supporting the body of FIG. 2and centro-symmetrically rotating the body while translating the bodyduring further processing.

FIG. 8 is a top planar view partially broken away of apparatusillustrated in FIG. 7 employed to centro-symmetrically rotate the bodyof FIG. 2 while that body translates.

FIG. 9 is a top planar view partially broken away of the translation ofthe body of FIG. 2 and the combined translation and centro-symmetricrotation of a plurality of the bodies of FIG. 2 in accordance with thisinvention and one type of heat source which may be used to heat thebodies.

FIGS. 10, 11, 12 and 13 are side elevation views partly in cross-sectionof bodies of semiconductor material processed in accordance with theteachings of this invention.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown the diamond cubic crystalstructure of silicon, silicon carbide, germanium, gallium arsenide, andother semiconductor materials consisting of a compound of a Group IIelement and a Group VI element and a compound of a Group III element anda Group V element.

In order to practice the new improved thermal gradient zone meltingprocess of this invention, it is necessary that the planar crystalorientation of one of the two major opposed surfaces of the wafer ofsemiconductor material be one selected from the group consisting of(100), (110) and (111). These preferred planar orientations, crystalaxis of migration, preferred crystal axis orientation of linear metaldeposits to be migrated, and the like are described in detail in ourrecently issued patents U.S. Pat. Nos. 3,899,362 and 3,904,442 theteachings of which are incorporated herein by reference and made a partof this application. A summary of the preferred crystal axes embodiedfor migrating metal "wires" is given in the following Table:

                  TABLE                                                           ______________________________________                                        Wafer  Migration  Stable Wire   Stable Wire                                   Plane  Direction  Directions    Sizes                                         ______________________________________                                        (100)  < 100 >           <  011 >*                                                                              < 100 microns                                                        < 0-11 >*                                                                              < 100 microns                               (110)  < 110 >           < 1-10 >*                                                                              < 150 microns                               (111)  < 111 >    a)     < 01-1 >                                                                      < 10-1 > < 500 microns                                                        < 1-10 >                                                               b)     < 11-2 >*                                                                     < -211 >*                                                                              < 500 microns                                                        < 1-21 >*                                                              c)     Any other                                                                     Direction                                                                              < 500 microns                                                        in (111)                                                                      plane*                                               ______________________________________                                         *The stability of the migrating wire is sensitive to the alignment of the     thermal gradient with the < 100 >, < 110 > and < 111 > axis, respectively     +Group a is more stable than group b which is more stable than group c.  

In addition, to migrating metal "wires" the improved process is alsosuitable for migrating any metal layer having a preferred geometricalshape such for example, as a grid structure for intersecting planarregions, annular shapes, square-like regions, hexagonal shapedconfigurations, disc-like areas and the like.

In order to describe the process in more detail, the semiconductormaterial is said to be silicon of N-type conductivity and the metal tobe migrated is said to be aluminum. These materials are chosen forillustrative purposes only without any intention of limiting the scopeof the present invention.

Referring now to FIG. 2, a body 10 of silicon semiconductor material,having N-type conductivity and a preferred level of resistivity, hasfirst and second major opposed surfaces 12 and 14, being respectivelythe top and bottom surfaces thereof. At least the first major surface 12has a preferred planar crystal orientation which is one of the groupconsisting of (100), (110) and (111) as described heretofore. The body10 also has a central axis which is substantially parallel with a firstpreferred axis of the crystal structure of the material andsubstantially perpendicular to the surfaces 12 and 14.

The body 10 is mechanically polished, chemically etched to remove anydamaged surfaces, rinsed in deionized water and dried in air. An acidresistant mask 16 is disposed on the first major surface 12 of the body10. Preferably, the mask is of silicon oxide which is either thermallygrown or vapor deposited on the surface 12 by any of the methods wellknown to those skilled in the art. Other suitable materials for thelayer of mask 16 are silicon nitride, aluminum nitride, aluminum oxideand the like. Employing well known photolithographical techniques, aphotoresist, such, for example, as Kodak Metal Etch Resist, is disposedon the surface of the silicon oxide layer 16. The resist is dried bybaking at a temperature of about 80° C. A suitable mask of the desiredgeometrical configuration, wherein the lines or disc-like areas are of apredetermined width or diameter is disposed on the layer of photoresistand exposed to ultraviolet light. After exposure, the layer ofphotoresist is washed in xylene to open windows in the mask where theone or more annular geometric configurations is desired so as to be ableto selectively etch the silicon oxide layer 16 exposed in the windows.

Selective etching of the layer 16 of silicon oxide is accomplished witha buffered hydrofluoric acid solution (NH₄ F--HF). The etching iscontinued until a second set of windows 18 corresponding to the windowsof the photoresist mask are opened in the layer 16 of silicon oxide toexpose selective portions of the surface 12 of the body 10 of silicon.The processed body 10 is rinsed in deionized water and dried. Theremainder of the photoresist mask is removed by immersion inconcentrated sulphuric acid at 180° C or by immersion in a mixture of 1part by volume hydrogen peroxide and 1 part by volume concentratedsulphuric acid.

Referring now to FIG. 2, a selective chemical etching of the exposedsurface area of the body 10 is accomplished with a mixed acid solution.The mixed acid solution is 10 parts by volume nitric acid, 70%, 4 partsby volume acetic acid, 100%, and 1 part by volume hydrofluoric acid,48%. At a temperature of from 20° to 30° C, the mixed solutionselectively etches the silicon of the body 10 at a rate of approximately5 microns per minute. An annular trough or trough-like depression 20 isetched in the surface 12 of the body 10 beneath window 18 of the oxidelayer 16. The selective etching is continued until the depth of thetrough 20 is approximately equal to the width of the window 18 in thesilicon oxide layer 16. However, it has been discovered that the trough20 should not be greater than approximately 100 microns in depth becauseundercutting of the silicon oxide layer 16 will occur. Undercutting ofthe layer 16 of silicon oxide has a detrimental affect on the width ofthe wire or molten zone to be migrated through the body 10. Preferably,a depth of 25 microns has been found suitable to practice the novelprocess. Etching for approximately 5 minutes at a temperature of 25° Cwill result in a trough 20 of from 25 to 30 microns in depth for a widthof the window 20 of from 10 to 500 microns. The etched body 10 is rinsedin distilled water and blown dry. Preferably, a gas such, for example,as freon, argon and the like, is suitable for drying the processed body10. The processed body 10 is disposed in a metal evaporation chamber. Alayer 22 of a suitable metal is deposited on the remaining portions ofthe layer 16 of silicon oxide and on the exposed silicon in the annulartrough 20. The metal in the trough 20 is the metal "wire" or mass to bemigrated or moved as a molten zone through the solid body 10. The metalof the layer 22 comprises a material, either substantially pure initself or admixed with one or more materials to impart a predeterminedsecond and opposite type conductivity to the material of the body 10through which it migrates. The thickness of the layer 22 isapproximately equal to the depth of the trough 20. Therefore, if thetrough 20 is 20 to 25 microns deep, the layer 20 is approximately 20 to25 microns in thickness. A suitable material for the metal layer 22 isaluminum to obtain P-type regions in N-type silicon semiconductormaterial. Prior to migrating the metal in the trough 20 through the bodyof silicon 10, the excess metal of the layer 22 is removed from thesilicon oxide layer 16 by such suitable means as grinding the excessmetal with a 600 grit carbide paper and photolithographical techniqueembodying selective chemical etching of the excess metal of the layer22.

It has been discovered that the vapor deposition of the layer 22 ofaluminum metal should be performed at a pressure of approximately 1 ×10⁻⁵ torr but not greater than 5 × 10⁻⁵ torr. When the pressure isgreater than 5 × 10⁻⁵ torr, we have found that in the case of aluminummetal vapor deposited in the trough 20, the aluminum does not penetrateinto the silicon and migrate through the body 10. It is believed thatthe layer of aluminum is saturated with oxygen and prevents reduction bythe aluminum metal of the very thin silicon oxide layer between thedeposited aluminum and the silicon that was formed in the air shortlyafter etching the troughs 18. Thus, the intial melt of aluminum andsilicon required for migration is not obtained because of the inabilityof aluminum to wet the silicon interface. In a like manner, aluminumdeposited by sputtering is not desirable as the aluminum appears to besaturated with oxygen from the process, thereby preventing the reductionof any intervening silicon oxide. The preferred methods of depositingaluminum on the silicon body 10 are by the electron beam method and thelike wherein little if any oxygen can be trapped in the aluminum.

With reference to FIGS. 3 and 4, the processed body 10 is placed in athermal migration apparatus, and the metal 22 in the trough 20 ismigrated through the body 10 as a molten zone by a thermal gradient zonemelting process. The body 10 is situated in the apparatus in a mannerwhereby a source of radiant energy 50 comprising a plurality of parallelquartz infrared lamps 55, 60, 65, 70, 75, 80, 85, 90, 95 and 100 spacedequally apart irradiates the body. The body 10 is mounted on a table 105in a manner whereby the bottom surface 14 of the body is maintained at atemperature level sufficient to establish and maintain a thermalgradient necessary for migration of the wire 22 through the body 10.Preferably, the table 105 rotates about its own vertical axis, or afirst center line, and the center line of the energy source ispreferably aligned with this first center line. The rotation of thetable 105 may be clockwise or counter clockwise.

In commercial processing as illustrated in FIG. 4, more than oneprocessed body 10 may undergo thermal gradient zone melting processingsimultaneously. The bodies 10 are arranged on the table 105 such thatpreferably all the vertical axes of the bodies are equidistant from thevertical axis of the table 105. As the table 105 rotates about itsvertical axis each of the bodies 10 of semiconductor material moves in acircular direction which, at any point of the bodies travel is thevector sum of movements parallel and perpendicular to the lamps.Therefore, by repeated rotations of table 105 during the temperaturegradient zone melting operation each body 10 will experience many randomtemperature irregularities in the radiation emitted by the lamps due tothe lamp structure and spacing but will experience each irregularity foronly an infinitesinal period of time. Therefore, each temperaturegradient experienced by a body will cause only a slight deflection orwiggle in the melt zone which ideally extends through the bodiesperpendicular to the major surfaces thereof. However, since during thetemperature gradient zone melting process table 105 will make a numberof revolutions, each body will be exposed to radiation emitted by thelamp array at a multiplicity of locations and will be exposed to amultiplicity of temperature irregularities. It has been discovered thatthe exposure of the bodies to many random irregularities for only briefperiods of time has the effect of each irregularity or lateral gradientto be effectively cancelled by an opposing irregularity or lateralgradient. Therefore, each wiggle or deflection in the melt zone iscancelled by a wiggle or deflection in an opposite direction giving theedges of each melt zone an extremely fine saw tooth configuration. Thesaw-tooth configuration is so fine that no adverse effects resulttherefrom and the devices produced by this method are characterized byabrupt, well defined junctions. For a source 50 of infrared energy at adistance of 2 centimeters from the top surface of table 105, it has beenfound desirable to rotate the table 105 at approximately 1 revolutionper minute.

An alternate means for moving bodies 10 to achieve the desired resultsis shown in FIGS. 5 and 6. Referring to FIGS. 5 and 6, there is shown aplurality of semiconductor bodies 10 rotating centro-symmetricallybeneath a source of radiant energy 50 the same as that illustrated inFIGS. 3 and 4. Each body 10 is supported by a rotatable platform 110fixed to a shaft 115 and supported by a stationary table 120. Shafts 115are rotatable by any suitable means such as gear sets or belt-pulleyarrangements (not shown) operatively connected to a motor or othersuitable prime mover. The centro-symmetric rotation of bodies 10 causeseach point on any one body to experience a multiplicity of randomtemperature irregularities and lateral temperature gradients resultingtherefrom, each for a brief period of time. This causes the melt zonesto take on fine saw tooth edge configurations due to the cancellingeffect of such lateral gradients as hereinabove set forth with respectto the method and apparatus illustrated in FIGS. 3 and 4. However, aswas the case with respect to the previously discussednoncentro-symmetric rotation of bodies 10, this saw tooth configurationis so fine that no adverse effects result therefrom and the devicesproduced by this method of centro-symmetric rotation possess the abruptwell defined junctions desired from a temperature gradient zone meltingprocess.

A second alternate means for moving bodies 10 to achieve the desiredresults of temperature gradient zone melting is shown in FIGS. 7, 8 and9. Referring to FIGS. 7, 8 and 9, there is shown a plurality ofsemiconductor bodies 125, 130 and 135. Each of the bodies shown in FIG.9 is supported by translating conveyor 140, causing the bodies totranslate in the same direction as the conveyor. The translation of thebodies in a direction skewed with respect to the parallel array of lampsor source of radiant energy 50 causes the bodies to experience amultiplicity of lateral temperature gradients due to the structure andspacing of the lamps substantially as previously described. However, thevelocity of conveyor 140 is such that these lateral gradients areexperienced by the semiconductor bodies for only brief periods of time.The exposure to a multiplicity of random lateral gradients for onlybrief periods of time causes these lateral gradients to cancel eachother giving the edges of the melt zones the fine saw toothconfiguration set forth hereinabove. This fine saw tooth configurationdefines for all practical purposes a straight melt zone edge providingthe abrupt well defined junctions desired by the temperature gradientzone melting process. In the preferred embodiment, the bodies aretranslated at an angle of between 30° and 60° from the axes of the lampsemployed in the heat source.

Should it be desired to make the junctions even more well defined, thewiggles in the melt zone edges can be made even finer bycentro-symmetrically rotating the semiconductor body while the bodytranslates. Such a combined semiconductor rotating and translatingmotion is experienced by bodies 125 and 130. Semiconductor bodies 125and 130 are supported by rotatable platforms 142, each fixed to a shaft145. Platforms 142 are supported by conveyor 140 apertured to receiveshafts 145. A stationary table 150 provides a supporting base forconveyor 140 and is slotted at 155 to accommodate shafts 145. Each shaft145 has a pinion 160 fixed to the end thereof. Pinion 160 is engageableby stationary rack 165 which causes the pinion and thus platform 142 torotate as conveyor 140 moves platform 142 in a rectilinear direction.The combined rotation and translation of semiconductor bodies 125 and130 causes these bodies to experience the random lateral temperaturegradients for even shorter periods of time than if the bodies were movedonly in translation thereby further enhancing the definition of the meltzones and thus the properties of the devices formed by the temperaturegradient zone melting process. Of course, if platforms 142 are ofsufficient diameter, a plurality of semiconductor bodies may be placedon each platform in the manner in which semiconductor bodies 10 areplaced on table 105 as illustrated in FIG. 4. In this arrangement, thesemiconductor bodies will experience a combined noncentro-symmetricrotation and translation.

A thermal gradient of approximately 50° C per centimeter between thebottom surface 14, which is the hot face and the surface 12, which isthe cold face, has been discovered to be appropriate when the apparatusoperating temperature is from 800° to 1400° C. The process is practicedfor a sufficient length of time to migrate or move, the metal layer as amolten zone through the solid body 10. For example, for an aluminum wireof 20 microns thickness, a thermal gradient of 50° C/cm, a 1200° C meantemperature of body 10 during processing, and at ambient or atmosphericpressure, a heating time of 10 minutes is required to migrate theannular wire 22 through the silicon body 10 having a thickness of 10mils.

The temperature gradient zone melting process and apparatus employed inthe process is not a part of this invention. Additionally, for a morethorough understanding of the temperature gradient zone melting processemployed in this invention and for a more thorough description of theapparatus employed for the process, one is directed to the copendingapplications and U.S. Patents of Anthony and Cline, which areincorporated herein by reference thereto, and entitled Method of MakingDeep Diode Devices, U.S. Pat. No. 3,901,736; High Velocity ThermalMigration Method of Making Deep Diodes, U.S. Pat. No. 3,910,801; DeepDiode Devices and Method and Apparatus, Ser. No. 552,154 filed Feb. 24,1975; High Velocity Thermomigration Method of Making Deep Diodes, U.S.Pat. No. 3,898,106; Deep Diode Device and Method U.S. Pat. No.3,902,925; and Stabilized Droplet Method of Making Deep Diodes HavingUniform Electrical Properties, U.S. Pat. No. 3,899,361.

Upon completion of the temperature gradient zone melting process, theresulting processed body 10 is as shown in FIG. 10. The thermalmigration of the metal in the troughs 18 as a molten zone through thebody 10 produces a processed body 10 having region 24 of a second andopposite type conductivity than the body 10. The material of the region24 is recrystallized semiconductor material of the body 10 suitablydoped with a material comprising the metal wire and having an impurityconcentration sufficient to obtain the desired conductivity. The metalretained in the recrystallized region is substantially the maximumallowed by the solid solubility of the metal in the semiconductormaterial through which it has been thermomigrated. It is recrystallizedmaterial having solid solubility of the metal therein. The region 24 hasa constant uniform level of impurity concentration throughout the entireregion of impart the predetermined type conductivity and predeterminedlevel of resistivity thereto. The width or diameter of the region 24 issubstantially constant throughout the entire region. In particular, whenthe body 10 is of silicon semiconductor material of N-type conductivity,the region 24 of aluminum doped recrystallized silicon forms therequired P-type conductivity region.

P-N junctions 26 and 28 are formed by the abutting contiguous surfacesof the material of opposite type conductivity. The P-N junctions 26 and28 are well defined and show an abrupt transition from one region ofconductivity to the next adjacent region of opposite type conductivity.The abrupt transition produces a step P-N junction. Laterally graded P-Njunctions 26 and 28 may be obtained by a post-diffusion heat treatmentof the region 24.

When region 24 is part of a grid structure or an annular shaped regionit encloses a region 30 of N-type conductivity material. The material isa part of the original material of the body 10. The region 24 functionsto electrically isolate the region 30 from the remainder of the body 10.

Alternately, the metal to be migrated may be sintered by a heattreatment process prior to forming the melt of metal-rich semiconductormaterial. An elevated temperature of from 500° to 550° C for a period oftime from 5 to 30 minutes will sinter the metal or disc to thesemiconductor material of the trough-like depressions. A preferredsintering temperature is 525° C ± 5° C for a period of time of about 20minutes. The process produces incipient fusion of a portion of the metalto the semiconductor material of the surface with which the metal is incontact. This sintering operation aids in preventing surface tensionfrom pulling the melt of metal-rich semiconductor material apart intotwo or more segments prior to initiation of the migration of the meltinto the solid body.

For a more detailed description of the sintering operation, one isreferred to the teachings of copending patent application Ser. No.645,672 filed Dec. 31, 1975, now U.S. Pat. No. 4,006,040 in the names ofMike F. Chang, Thomas R. Anthony and Harvey E. Cline and entitledSemiconductor Device Manufacture. This application is incorporatedherein by reference.

Referring now to FIG. 11, the process might alternately be practiced byutilizing the oxide layer 16 as mask to restrain the lateral flow of themetal when the melt is formed. The metal 22 may be sintered by a processdescribed heretofore or may be migrated without sintering.

This alternate procedure often does not retain the dimensional stabilityof the configuration of the metal when it becomes molten. Some flow ofmelt of the metal-rich semiconductor material may occur under the layer16 of the interface with surface 12 in the vicinity of the window 18.However, if dimensional stability is not critical then this alternateprocess may be practiced.

For a more detailed teaching of the employment of the layer 16 as ameans to aid in initiating the thermal gradient zone melting process,one is directed to copending patent application Ser. No. 634,247, filedNov. 21, 1975 in the names of Mike F. Chang, Thomas R. Anthony andHarvey E. Cline and entitled Method For Thermigration of Selected MetalsThrough Bodies of Semiconductor Material. This application isincorporated herein by reference.

This improved process of this patent application is suitable for thefabrication of planar or mesa devices in the region 30 when a gridstructure or annular structure is produced. However, it is sometimesdesirable that the region 30 be electrically isolated from the surface14 as well. Referring now to FIG. 12, a region 32 of the same typeconductivity as that of region 24 and opposite to that of region 30 isformed in the body 10. The region 32 may be formed by epitaxial growth,diffusion and the like to form a cell comprising the electricallyisolated region 30. A P-N junction 34 is formed by the abuttingcontiguous surfaces of the material of regions 30 and 32 of oppositetype conductivity. An electrical device 36 such, for example, as aplanar SCR is obtained after the forming of the regions 24 and 32.

Referring now to FIG. 13, the novel process may be practiced wherein theregion 24 only extends to a predetermined distance from surface 12 intothe body 10. A semiconductor device 40, such as a diode and the like,may be formed in the area of several intersecting regions 24. The device40 may be formed before or after the formation of the regions 24.

For a more thorough discussion of the process of migrating metal layersas a melt part way into a solid body of semiconductor material, one isdirected to the copending application Ser. No. 559,262 filed Mar. 17,1975, and now U.S. Pat. No. 3,988,770 in the names of Thomas R. Anthonyand Harvey E. Cline, and entitled Deep Finger Diodes, the teachings ofwhich are incorporated herein by reference thereto.

This invention is related to U.S. patent application Ser. No. 733,239filed of even date herewith in the names of Harvey E. Cline and ThomasR. Anthony and entitled "Apparatus For Imparting CombinedCentro-Symmetric and Noncentro-Symmetric Rotation to SemiconductorBodies". This application is assigned to the assignee of the presentinvention and discloses and claims an apparatus which imparts aplanetary rotation to semiconductor bodies while the bodies undergoprocessing by temperature gradient zone melting.

What is claimed is:
 1. An improved method for migrating a molten zonethrough a solid body of semiconductor material comprising the processsteps of:(a) selecting a body of single crystal semiconductor materialhaving two major opposed surfaces, a predetermined type conductivity, apredetermined level of resistivity, a preferred diamond cubic crystalstructure, a preferred planar crystal orientation for at least a firstmajor surface, said orientation being one selected from the groupconsisting of (100), (110) and (111), and a first preferred crystal axisand a central axis which are each substantially perpendicular to atleast the first major surface and substantially parallel with eachother; (b) depositing a layer of a metal of a predetermined thicknessand having a predetermined geometrical configuration on the first majorsurface having the preferred planar orientation; (c) supporting the bodyon a support; (d) rotating the body about its own central axis; (e)heating the body and the deposited metal to a predetermined elevatedtemperature with a heat source, said elevated temperature beingsufficient to form a melt of a metal-rich semiconductor material on thefirst major surface of the body while continuing the rotation of thebody, said heat source comprises a parallel array of lamps and the bodyis translated in a plane parallel to that of said array of lamps at anangle of between 30° and 60° to the central axes of said lamps; (f)establishing a temperature gradient substantially parallel with thecentral axis of the body and the first preferred crystal axis of thecrystal structure of the material while continuing the rotation of thebody, the first major surface on which the melt is formed being at thelower temperature; (g) translating the body through a distance heated bysaid heat source while continuously rotating the body, and (h) migratingeach melt of metal-rich semiconductor material as a molten zone throughthe solid body of semiconductor material for a sufficient period of timeto reach a predetermined distance into the body from the first majorsurface, while continuing the rotation of the body, to form in situ atleast one region of recrystallized semiconductor material of the bodyhaving solid solubility of the deposited metal therein, a substantiallyuniform cross-sectional area and a substantially uniform level ofresistivity throughout the entire region.
 2. The method of claim 1whereinthe semiconductor material of the body is one selected from thegroup consisting of silicon, silicon carbide, germanium and galliumarsenide.
 3. The method of claim 2 including, prior to depositing themetal, the additional process step of:depositing a layer of a materialwhich is one selected from the group consisting of silicon oxide,silicon nitride, aluminum oxide and aluminum nitride on the first majorsurface having the preferred crystal planar orientation, and etchingselectively the layer of material to open at least one window therein toexpose a predetermined surface area of the body defining the geometricalconfiguration for the metal to be deposited therein.
 4. The method ofclaim 3 whereinthe semiconductor material is silicon of N-typeconductivity, and the metal is aluminum.
 5. The method of claim 3whereinthe layer of metal is heated to a predetermined elevatedtemperature to sinter a portion of the metal with the semiconductormaterial in contact therewith.
 6. The method of claim 1 including, priorto depositing the metal layer, the additional process step of:etchingselectively the first major surface having the preferred crystal planarorientation to form at least one trough-like depression therein having apreferred geometrical configuration.
 7. The method of claim 6 whereinthesemiconductor material of the body is one selected from the groupconsisting of silicon, silicon carbide, germanium and gallium arsenide.8. The method of claim 7 including, prior to depositing the metal, theadditional process step of:depositing a layer of a material which is oneselected from the group consisting of silicon oxide, silicon nitride,aluminum oxide and aluminum nitride on the first major surface havingthe preferred crystal planar orientation, and etching selectively thelayer of material to open at least one window therein to expose apredetermined surface area of the body defining the geometricalconfiguration for the metal to be deposited therein.
 9. The metod ofclaim 8 whereinthe semiconductor material is silicon of N-typeconductivity, and the metal is aluminum.
 10. The method of claim 9whereinthe layer of aluminum is substantially oxygen free.
 11. Themethod of claim 8 whereinthe layer of metal is heated to a predeterminedelevated temperature to sinter a portion of the metal with thesemiconductor material in contact therewith.
 12. The method of claim 1including the additional process step offorming on the body a planarlayer of semiconductor material of a conductivity type opposite to thatof the material of the body, said planar layer having two opposed majorsurfaces substantially parallel to each other and to the major opposedsurfaces of the body, a major surface of the planar layer beingcoextensive and contiguous with the second major surface of the body,and wherein each melt is migrated a predetermined distance through thebody from the first major surface to at least intersect, and be integralwith, the planar layer and to form a cell of material of the bodyelectrically isolated from the remaining material of the body and fromthe second major surface of the body.
 13. The method of claim 1including the additional process step offorming a planar layer ofsemiconductor material on the second major surface of the body, thematerial of the planar layer having opposed major surfaces which aresubstantially parallel with each other and to the opposed major surfacesof the body, and wherein each melt is migrated a predetermined distancethrough the body from the first major surface to at least intersect andbe integral with the planar layer and to form a cell of material of thebody electrically isolated from the remaining material of the body andfrom the second major surface of the body.
 14. An improved method formigrating a molten zone through a solid body of semiconductor materialcomprising the process steps of:(a) selecting a body of single crystalsemiconductor material having two major opposed surfaces, apredetermined type conductivity, a predetermined level of resistivity, apreferred diamond cubic crystal structure, a preferred planar crystalorientation for at least a first major surface, said orientation beingone selected from the group consisting of (100), (110) and (111), and afirst preferred crystal axis and a central axis, each of which axesbeing substantially perpendicular to at least the first major surfaceand substantially parallel with each other; (b) depositing a layer of ametal of a predetermined thickness and having a predeterminedgeometrical configuration on the first major surface having thepreferred planar orientation; (c) supporting the body on a supportingsurface; (d) translating said body through a distance heated by a heatsource; (e) heating the body and the deposited metal to a predeterminedelevated temperature with said heat source, said elevated temperaturebeing sufficient to form a melt of a metal-ring semiconductor materialon the first major surface of the body while continuing the translationof the body, said heat source comprises a parallel array of lamps andthe body is translated in a plane parallel to that of said array oflamps at an angle of between 30° and 60° to the central axes of saidlamps; (f) establishing a temperature gradient substantially parallelwith the central axis of the body and the first preferred crystal axisof the crystal structure of the material while continuing thetranslation of the body, the first major surface on which the melt isformed being at the lower temperature, and (g) migrating each metal ofmetal-rich semiconductor material as a molten zone through the solidbody of the semiconductor material for a sufficient period of time toreach a predetermined distance into the body from the first majorsurface while continuing the translation of the body, to form in situ atleast one region of recrystallized semiconductor material of the bodyhaving solid solubility of the deposited metal therein, a substantiallyuniform crosssectional area and a substantially uniform level ofresistivity throughout the entire region.
 15. The method of claim 14whereinthe semiconductor material of the body is one selected from thegroup consisting of silicon, silicon carbide, germanium and galliumarsenide.
 16. The method of claim 15 including, prior to depositing themetal, the additional process step of:depositing a layer of a materialwhich is one selected from the group consisting of silicon oxide,silicon nitride, aluminum oxide and aluminum nitride on the first majorsurface having the preferred crystal planar orientation, and etchingselectively the layer of material to open at least one window therein toexpose a predetermined surface area of the body defining the geometricalconfiguration for the metal to be deposited therein.
 17. The method ofclaim 16 whereinthe semiconductor material is silicon of N-typeconductivity, and the metal is aluminum.
 18. The method of claim 16whereinthe layer of metal is heated to a predetermined elevatedtemperature to sinter a portion of the metal with the semiconductormaterial in contact therewith.
 19. The method of claim 14 including,prior to depositing the metal layer, the additional process stepof:etching selectively the first major surface having the preferredcrystal planar orientation to form at least one trough-like depressiontherein having a preferred geometrical configuration.
 20. The method ofclaim 13 whereinthe semiconductor material of the body is one selectedfrom the group consisting of silicon, silicon carbide, germanium andgallium arsenide.
 21. The method of claim 20 including, prior todepositing the metal, the additional process step of:depositing a layerof a material which is one selected from the group consisting of siliconoxide, silicon nitride, aluminum oxide and aluminum nitride on the firstmajor surface having the preferred crystal planar orientation, andetching selectively the layer of material to open at least one windowtherein to expose a predetermined surface area of the body defining thegeometrical configuration for the metal to be deposited therein.
 22. Themethod of claim 21 whereinthe semiconductor material is silicon ofN-type conductivity, and the metal is aluminum.
 23. The method of claim21 whereinthe layer of metal is heated to a predetermined elevatedtemperature to sinter a portion of the metal with the semiconductormaterial in contact therewith.
 24. The method of claim 20 whereinthelayer of aluminum is substantially oxygen free.
 25. The method of claim14 including the additional process step offorming on the body a planarlayer of semiconductor material of a conductivity type opposite to thatof the material of the body, the planar layer having two opposed majorsurfaces substantially parallel to each other and to the major opposedsurfaces of the body, a major surface of the planar layer beingcoextensive and contiguous with the second major surface of the body,and wherein each melt is migrated a predetermined distance through thebody from the first major surface to at least intersect, and be integralwith, the planar layer and to form a cell of material of the bodyelectrically isolated from the remaining material of the body and fromthe second major surface of the body.
 26. The method of claim 14including the additional process step offorming a planar layer ofsemiconductor material on the second major surface of the body, thematerial of the planar layer having opposed major surfaces which aresubstantially parallel with each other and to the opposed major surfacesof the body, and wherein each melt is migrated a predetermined distancethrough the body from the first major surface to at least intersect andbe integral with the planar layer and to form a cell of material of thebody electrically isolated from the remaining material of the body andfrom the bottom surface of the body.